Various types of systems (e.g., communication systems, sonar imaging systems, etc.) use transmission of energy (e.g. electromagnetic energy, acoustic energy, etc.) though a transmission medium (e.g., air, water, etc.). Some transmission media have a frequency dependent loss characteristic. In some applications (e.g., narrowband applications) this frequency dependent loss characteristic has little effect on system performance. However, in other applications (e.g., wideband applications) this frequency dependent loss characteristic can negatively impact system performance.
Acoustic energy, i.e. sound, and particularly sound propagating in water, is known to have a particularly strong relationship between sound frequency and sound intensity (power per unit area) versus range from a transmitting source. Higher frequency sound loses intensity with range more than low frequency sound.
There is a desire to improve the resolution of systems that transmit energy, for example, particular, acoustic imaging systems. Accordingly, there has been a desire to develop new systems that use larger apertures in order to increase azimuthal resolution and that use higher bandwidth pulses in order to increase range-wise resolution.
The use of wideband transmissions, followed by pulse compression using a matched filter, provides a resolution equal to BW/(2*C), where BW is the bandwidth of the pulse and C is the speed of the wave in the medium.
As sound travels outward through a medium, e.g., water, intensity (power per unit area) of the sound is reduced. The reduction in intensity is due to a variety of propagation loss factors, including, but not limited to, spreading loss and absorption loss. For spreading loss, intensity of the sound is reduced with range as the sound increasingly spreads (e.g., spherically) with increasing range. For absorption loss, intensity of the sound is reduced with range due to heating of the medium associated with molecular action of the medium.
Spreading loss is a function of spreading geometry of the projected sound and is frequency independent. In contrast, absorption loss can be highly dependent on the frequency of the energy, for example, sound travelling through the medium and also depends on properties of the medium itself, e.g., density, temperature, salinity, etc.
Sound travelling in water is used in examples herein. However, the same or similar apparatus and techniques can apply to any energy, with sufficient bandwidth, travelling through a lossy medium to a range, resulting in a frequency-dependent loss at the range within the bandwidth.
Referring now to FIG. 1, a graph 100 has a horizontal axis with a log scale in units of frequency in cycles per second (i.e., Hertz), and a vertical axis with a log scale in units of absorption loss in dB per kiloyard. Absorption loss curves 102, 104, 106 show losses for an acoustic signal as a function of frequency for sound traveling in the ocean (salt water) at three different water temperatures, five degrees Celsius, fifteen degrees Celsius, and 22.5 degrees Celsius, respectively.
As can be seen from the absorption curves 102, 104, 106, at lower frequencies, the absorption coefficient is smaller and changes less rapidly with frequency.
Thus, for narrowband signals, at lower frequencies, and for shorter ranges, it is reasonable to make the assumption that the absorption coefficient (and absorption loss) across the band of the transmission frequencies is approximately the same, and is also small. At the lower frequencies, it will be appreciated that the primary cause of sound propagation loss is spreading loss and not absorption loss.
As can also be seen from the absorption curves 102, 104, 106, at higher frequencies, and for wider bandwidth signals, absorption loss can be more dominant than spreading loss, and the absorption loss can vary across the band of transmission frequencies.
The net effect of a propagation path dominated by absorption loss, where the absorption coefficient varies greatly across a transmitted signal within a frequency band of interest from a lower frequency to a higher frequency, is that, an echo returned from a target at range in response to a transmitted signal is not white (i.e., not flat with frequency, i.e., colored). From FIG. 1 it should be apparent that, amplitude of frequency content of the received (or return echo) signal is higher towards the lower frequency end of a frequency band of interest because of the differences in absorption between low and high frequencies.
The longer the propagation path, the more exacerbated this effect becomes. Thus, lower frequency transmitted signals travelling greater ranges will experience a similar effect as higher frequency transmitted signals travelling shorter ranges.
As described above, it is desirable to use wider bandwidth signals and to use pulse compression in order to improve range-wise resolution. However, if the propagation path adds significant acoustic color (non-whiteness) to the received (or return echo) signal, then pulse compression, for example, pulse compression that uses cross correlation of the received signal with a replica related to the transmitted signal (i.e., replica correlation), even using a replica that is compensated for the frequency-dependent loss in the medium, will not allow recovery of a fully pulse compressed signal, and the range-wise resolution will be degraded.